There are several known allergens produced by Aspergillus and other fungi that have a similar effect, and similar to dander and dust mite they are very difficult to avoid. These allergens are therefore very important medically - consequently there is an allergen database storing information on them for research use here: Allergome - Allergome recently celebrated its 10 year anniversary.
Many allergens are known to be proteins but we also know that there are are huge variety of proteins that we can inhale every day that do not cause allergy. Allergens seems to have little in common in terms of size. There are allergens that cause strong allergic responses that are large in size and equally effective small allergens. It is not known why some proteins make powerful allergens and others do not, we do not know enough about how a particular protein triggers the allergic cascade of events in our airways to be able to explain allergy adequately.
Recent research shows that we may be starting to find our some of the answers to these unknowns. Herre et.al. have found that microbial lipopolysaccharide (LPS) interacts with cat dander allergen and that this interaction enhances the allergic response. In a home with either cat dander or lots of bacterial growth (e.g. a damp home) there may not be much of an allergic reaction caused, but both together is more likely to cause health problems. This is a new type of allergy initiation mechanism and points the way to the development of new drugs (or possibly new home cleaning agents) that may prevent the interaction between these substances and thus cut allergy.
There are other types of interaction known to contribute to allergy - it is interesting to note that here we have one mechanism that might help explain why people living in damp homes are prone to allergy. There is a natural inclination to blame health problems in damp homes on visible signs of dampness such as the growth of moulds, but here is an example of ill health caused by invisible components of damps homes e.g. bacterial growth.
Story on BBC website